Virtually all aircraft variable pitch propellers are of the constant speed type and are most commonly used for engine speed synchronization on multi-engined aircraft. Certain single-engined aircraft also use this propeller because of its availability but could otherwise use a simple variable pitch propeller.
To date, no hydraulically operated propeller has been developed to be as safe and practical as the foregoing constant speed type whereby engine oil is internally supplied through a special transfer bearing within the engine crankcase, then through a hollow bored propeller shaft to the propeller hub.
Hydraulic fluid transmitted from any external source through a rotating seal on the propeller shaft to the propeller hub is a gross misapplication due to continuous seal wearout, potential external leaking and pressure limiting characteristics.
Accordingly, the following mechanical impulse force method of powering and controlling a self-contained hub hydraulic system to operate the propeller is a simple and practical means of operating a propeller without external fluid connections and could be adapted to thousands of single-engined aircraft now not capable of modernizing such aircraft for improved performance.